This invention relates to a method and apparatus for maintaining relative positional alignment between two spaced-apart points on a moveable member. The method and apparatus disclosed and claimed below has a wide range of application. The particular embodiment disclosed and described in this application relates to the use of the apparatus and method on a driven tail pulley which supports one end of an endless bucket conveyor belt such as is used in grain elevators. The apparatus and method according to the present invention enables the belt to be positioned and maintained in the precise center of the tail pulley. The present invention also permits the maintenance of proper tension on the moving conveyor belt, thereby preventing slippage between the belt and the drive pulley.
The conveyor belt and associated equipment referred to in the embodiment described below are contained in what is referred to in the grain storage industry as a "head house." The conveyor belt has buckets affixed to it in spaced-apart relation which are used to carry the product from a storage bin upwardly to the top of a grain elevator complex, where the product is then directed into a selected storage elevator. The conveyor belt is driven by a head pulley located in the top of the head house. The pulley is driven by a large electric motor with a gear reduction apparatus which reduces the rpm of the head pulley to between 20 and 50 revolutions per minute. The linear speed of the moving conveyor belt may be between 180 and 245 meters per minute and the overall height of the conveyor, from 30 to 90 meters. The tail pulley is positioned in the lower portion of the head house in a box-like metal container called a "boot" and is driven by the movement of the conveyor belt around its periphery.
In recent years, federal health and safety laws have required that a shroud of duct work be installed closely around the moving belt in order to enclose it and prevent the build up of dust in the head house. This has been done not only because of the perceived danger to employees of breathing dust-laden air, but also to prevennt violent explosions caused by dust-laden air being ignited by heat or sparks.
While the erection of this shroud completely around the belt path does reduce the build up of dust in the surrounding environment, the air inside the shroud in close proximity to the moving conveyor belt is saturated with dust. It is therefore imperative to prevent sparks or the build up of heat within this shroud.
Heat build-up can be minimized by maintaining proper tension on the belt. There are many factors which can cause variation in the tension of the belt around the rotating head and tail pulleys. For example, loading of the buckets on the conveyor belt with product greatly increases the weight on the belt, causing it to stretch. The lack of sufficient tension causes the belt to slip as it passes around the head pulley. The relative movement between the conveyor belt and the head pulley causes friction and the build up of heat.
In order to prevent this slippage, the tail pulley is mounted on what is referred to as a "take-up bearing," wherein each end of the tail pulley axle is rotatably positioned in a bearing block, and mounted, respectively, for adjustable sliding movement within opposing slots in the boot. As required, the bearings on both sides of the take-up pulley are weighted in order to provide tension on the head and tail pulleys around which the belt moves. Heretofore, any tension adjustment has usually been made manually.
Because the conveyor belt is enclosed within the shroud, it is particularly important to maintain the rotating conveyor belt in its proper position in the center of the tail pulley, and to prevent the belt from moving from side to side. As mentioned above, the linear speed of the moving conveyor belt may be as much as 180 to 245 meters per minute. Any rubbing of the belt against the interior walls of the shroud in an atmosphere laden with dust can cause a rapid build up of heat or a spark which causes explosive ignition of the dust.
It is known that in order for a conveyor belt to move around an end pulley without lateral, or side-to-side, movement, the axis of rotation of the pulley axle must be 90.degree. to the center line of the belt. Since, as already discussed, conveyor belts are subject to a variety of factors which can cause variation in belt tension, including the necessity of independently adjusting the take-up bearing on one side or the other of the tail pulley in order to keep the conveyor belt centered, the solution lies in establishing and maintaining relative positional alignment between two spaced-apart points on the pulley axle. In the preferred embodiment disclosed below, these two spaced-apart points are defined as first and second take-up bearings mounted on the pulley for independent planar movement. As is therefore apparent, an adjustment necessary to keep the conveyor belt centered may have an adverse effect on belt tension, and vice versa.
Numerous attempts have been made in the past to devise machines for automatically centering an endless band, such as a conveyor belt, trained over two spaced-apart rolls. For example, the Repetto U.S. Pat. No. 4,173,904 discloses such an apparatus which, as in many other prior art patents, senses lateral movement by means of levers positioned near the opposite side edges of the moving belt which are tripped by lateral off-center movement of the belt. In Repetto, the band is automatically centered by two hydraulic cylinder-piston units. The first unit displaces one end of the stretching roll in one plane, and the second cylinder simultaneously displaces the other end in a plane perpendicular to the first.
The Promin U.S. Pat. No. 3,603,451 discloses side-positioned sensors which move a transverse idler roller on which the belt is supported angularly in a plane parallel to that of the direction of belt movement and in a direction to recenter the belt. The sensors comprise rollers adapted to rotate when the adjacent side edge of a moving belt comes into contact with it. The rotation of the sensing roller by this contact action drives a hydraulic pump which pumps fluid to a cylinder connected to the idler roller which then moves the idler roller in such direction as to position the belt in its center.
The Dyke U.S. Pat. No. 3,159,268 discloses another use of side sensors, which are positioned on the same side of the belt and control the upwardly or downwardly movement of an adjusting screw.
As is recognized in the Begiebing U.S. Pat. No. 3,489,264, overshooting of the belt from one side of the roller to the other as hydraulic correction takes place is a problem which has been inherent in most previous belt-training mechanisms. In Begiebing, a mechanism is disclosed which damps the "hunting" tendency of belt-training mechanisms which are susceptible to continual over-correction in alternate directions as the belt continuously moves from side to side on the roller.
Insofar as is known, no prior belt training apparatus is capable of simultaneously maintaining precise tension on a rotating conveyor belt, while maintaining the belt in the center of the rotating pulley without over-correction from side to side.